1. Field of the Invention
The present invention relates to a spin-tunnel transistor, which may be used as a magnetic sensor such as a magnetic head for reproduction of high-density magnetic memory or a magnetic memory such as a random access memory.
2. Discussion of the Related Art
The development of ferromagnetic reproducing heads promoted high-density and high-speed information technology. For example, the development of Giant MagnetoResistance effect device (GMR device) for high-power ferromagnetic reproducing increased a recording density of magnetic medium of annual rate by 100%.
The GMR device has a sandwich-structured laminate film including a ferromagnetic metal layer, a nonmagnetic metal layer and a ferromagnetic metal layer. The laminate film is often referred to as a “spin valve film”. In this spin valve film, one ferromagnetic metal layer has a substantially pinned magnetization, which is fixed by a ferromagnetic coupling bias applied thereto by an exchange coupling between the ferromagnetic metal layer and an antiferromagnetic metal layer formed adjacent to the ferromagnetic metal layer. The magnetization pinned layer of ferromagnetic metal material may be referred to as “a magnetization pinned layer”, while another ferromagnetic metal layer has a magnetization free to rotate in an applied magnetic field (usually signal magnetic field) is usually referred to as “a magnetization free layer”.
When a magnetic field is applied to the spin valve film, relative angles between the magnetization directions of the two ferromagnetic metal layers changes, and the resistance of the spin valve changes by the change of the relative angle. For example, the resistance of the spin valve film becomes the smallest when the two magnetizations are antiparallel to each other, while the resistance of the spin valve film becomes the largest when the two magnetizations are parallel to each other. A voltage or current detector is usually coupled to the spin valve film and senses the resistance change of the spin valve film by detecting the current or voltage change between two terminals of the spin valve film, whereby a change of applied magnetic field being detected.
A TMR (Tunneling MagnetoResistance) device is also being developed for magnetic recording of higher density. The TMR device has a laminate film of a ferromagnetic metal layer, a tunnel barrier layer and a ferromagnetic metal layer, in which a voltage is applied between these ferromagnetic metal layers. A resistance change between the two ferromagnetic metal layers is detected by sensing a change of tunnel current flowing through the three layers. In the TMR device, the tunnel current varies depending on the relative angle of two magnetizations of the ferromagnetic metal layers, therefore a change of applied magnetic field can be sensed by detecting the tunnel resistance change.
A three-terminal spin valve transistor, which is different from two-terminal sensors like TMR device and GMR device, is also being developed. The three-terminal spin valve transistor comprises an emitter, a base and a collector, and the base therein may have several ferromagnetic metal layers and nonmagnetic metal layer formed between the ferromagnetic metal layers. The emitter and the collector of the three-terminal spin valve transistors may have semiconductor, whereby forming a transistor of SMS (Semiconductor-Metal-Semiconductor) structure, where the metal base is sandwiched by the emitter and collector. Also, transistors of MIMS (Metal-Insulator-Metal-Semiconductor) structure and MIMIM (Metal-Insulator-Metal-Insulator-Metal) structure are also proposed. Each of the MIMS and MIMIM structures comprises a tunnel barrier layer of a dielectric insulator material between the emitter and the base or between the base and the collector. Based on a spin-tunnel phenomenon in the Metal/Insulator/Metal junction, they are a type of spin valve transistor having the MIM structure may be referred to as spin-tunnel transistor.
In the spin-tunnel transistor, hot electrons are formed by current flow at the tunnel junction and they scatter in the base caused by its spin direction. The base usually has two ferromagnetic metal layers and a nonmagnetic metal layer between the two ferromagnetic metal layers. The hot electrons' scatter becomes the largest when a relative angle between two metal layers of ferromagnetic material in the base becomes the largest (both of magnetization directions are antiparallel to each other), whereby decreasing the collector current. When the relative angle between the two metal layers of ferromagnetic material in the base becomes the smallest (both of the magnetization directions are parallel to each other), collector current increases. The direction of signal magnetic field can be detected by sensing the change of the collector current,
One of the two metal layers of ferromagnetic material may have a magnetization free to rotate in an applied magnetic field, the other of the two metal layers of ferromagnetic material may have a magnetization fixed in the applied magnetic field, and the interposed nonmagnetic layer decouples the magnetic coupling between the two ferromagnetic metal layers, thereby forming a spin valve structure with the interposed nonmagnetic metal layer.
Examples of the spin-tunnel transistors are disclosed in a prior U.S. patent application Ser. No. 10/400,569, the entire contents of which are incorporated herein by reference.